Researchers found people with gum disease who ate two grapefruit a day for a fortnight showed significantly less bleeding from the gums.

They believe this is due to an increase in blood levels of vitamin C, known to promote wound healing and cut damage by unstable free radical molecules.

The research, by Friedrich Schiller University in Germany, is published in the British Dental Journal.

The study of 58 people with chronic gum disease found that eating grapefruits had a positive effect on both smokers and non-smokers.

It reinforces the message that if you have enough vitamin C in your diet then it tends to promote healing

Dr Gordon Watkins

Smoking is known to increase the risk of gum disease.

At the start of the two-week study, virtually all those taking part had low levels of vitamin C in their blood plasma.

On average smokers' vitamin C level was 29% lower than that for non-smokers. Eating two grapefruit a day raised vitamin C plasma levels for all those who consumed them.

In smokers, the level almost doubled, but because they started from a lower baseline their average vitamin C level was still lower than that recorded in the non-smokers.

It is unclear why smoking is associated with lower vitamin C levels. It is possible that it alters the way the body metabolises the vitamin, but it is also possible that smokers tend to have a less healthy diet.

Tooth loss

Dr Gordon Watkins, a scientific advisor to the British Dental Association, told the BBC News website that gum disease was a major problem in the UK.

"Most people have a small amount of gum disease, much of which is reversible," he said.

"But a significant amount have intractable gum disease, which is the biggest cause of loss of teeth in adults.

"This is a small-scale, short-term study but it reinforces the message that if you have enough vitamin C in your diet then it tends to promote healing."

Dr Watkins said that a healthy, balanced diet should provide enough vitamin C.

However, he stressed that the body was unable to store up excess amounts of the vitamin, so a steady intake was important.

Each grapefruit contains approximately 92.5 mg of vitamin C.

Those taking part in the research were advised not to brush their teeth immediately after consuming the grapefruits.

This is because citrus fruits are acidic and can weaken tooth enamel making it susceptible to erosion.